Recently, desalting seawater through a composite semipermeable membrane has been tried, and now, it has been put into practical use in water plants all over the world. A composite semipermeable membrane generally comprises a separating functional layer coated on a porous substrate film. When the separating functional layer is formed by using crosslinked aromatic polyamide, there are such advantages that the layer is stiff and rigid since it contains a benzene ring and that the layer can be readily formed by interfacial polycondensation of aromatic polyfunctional amine and aromatic polyfunctional acid halide, and other advantages such as a high salt removal ratio and a high permeation flow rate are known (JP-A-1-180208 and JP-A-2-115027).
However, the water quality standard in the art is controlled more and more severely these days. In particular, it is difficult to reduce the minor boron in seawater to a level acceptable for drinking water in ordinary treatment, and some composite semipermeable membranes have been proposed for solving this problem (JP-A-11-19493 and JP-A-2001-259388). However, since these membranes are expected to have a membrane permeation flow rate of 0.5 m3/m2·day or less and a boron removal ratio at most about 91 to 92% when seawater at 25° C. having a pH of 6.5, a boron concentration of 5 ppm and a TDS concentration of 3.5% by weight is permeated under an operation pressure of 5.5 MPa, development of composite semipermeable membranes which have higher solute-rejection performance has been desired.
In order to improve the solute-rejection performance of a composite semipermeable membrane, it might be considered to make the pore size of the separating functional layer of the composite semipermeable membrane small. However, at the same time, to achieve sufficient water permeation capability, it is necessary to make the pore size appropriately large. On the other hand, it is considered that, even if the pore size in the separating functional layer of the composite semipermeable membrane is small, when the pore number and the vacancy content in the semipermeable membrane are large, the permeated water volume increases but the solute-rejection performance deteriorates. In other words, in order to improve the solute-rejection performance and maintain the permeated water volume, both of the pore size and the vacancy content in the separating functional layer of the composite semipermeable membrane must be appropriately regulated. As one means for measuring the pore size distribution and the vacancy content in a thin membrane, there is conventionally disclosed a method using positron annihilation lifetime spectroscopy. (JP-A-2001-142233 and JP-A-11-169689). But, since the correlation between the pore radius as well as the vacant content and the solute removal ratio has not been found with these method, an indicator representing the boron removal performance of the composite semipermeable membrane has not been obtained so far.
As means for improving the solute-rejection performance of composite semipermeable membranes, a method in which a novel reactant is added to the reaction solution is exemplified. Since this method does not so much differ from conventional methods, it is useful as a simple improved method. For example, it is described that in the composite semipermeable membrane, crosslinked polyamide is constituted by a polyamine component having at least two amino groups in the molecule, and an acid component, as a novel reactant, which comprises a linear aliphatic poly-acid halide having at least two halogenocarbonyl groups in the molecule (Japanese Patent 3,031,763). Although it is described that the method provides a composite semipermeable membrane having a high salt removal ratio and a high permeation flow rate and a production process thereof, it is still impossible to further increase the boron rejection ratio according to the method.